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The relationship between the size, activity and electronic character of catalysts

Chemists from the University of Utah have reported the first unambiguous relationship between the size of catalyst particles on a solid substrate, their electronic properties and their ability to accelerate chemical reactions.

Chemists from the University of Utah have reported the first unambiguous relationship between the size of catalyst particles on a solid substrate, their electronic properties and their ability to accelerate chemical reactions. The research is another step towards the goal of designing cheaper and more efficient catalysts that will increase energy production, reduce greenhouse gas emissions and assist in the production of a variety of products in the fields of medicine and oil.

Catalysts are substances that speed up chemical reactions without being consumed during the reaction. They are used in the production of most chemicals and many industrial products and the world economy definitely depends on them.

"One of the most significant uncertainties related to catalysts lies in the fact that we do not really understand how the size of the catalyst particles affects the occurrence of the chemical reaction," says Scott Anderson, professor of chemistry at the University of Utah and one of the authors of the article that appeared in the scientific journal Science. "If we can understand the factors that affect the activity of the catalysts - then we can prepare more efficient and cheaper catalysts."

"Most catalysts are expensive noble metals such as gold, palladium or platinum," he adds. "For example, for gold catalysts, most of the metal appears in the form of large particles, but these particles are not active and only nanoparticles, containing only about ten atoms, are active. That is, more than ninety percent of the gold in the catalyst does nothing and does not contribute to the catalytic reaction. If we can make a catalyst consisting only of the desired particle size, we can save about ninety percent or more of its price."

In addition, "there is increased interest in understanding how to produce effective catalysts from much less expensive metals such as copper, nickel and zinc," says the researcher. "And the way you will be able to do this lies in adjusting the chemical properties of the catalysts, that is, adjusting their electronic properties, because the electrons are the ones that control the chemistry."

The idea is to take a metal that is not catalytically active and by reducing it to the appropriate particle size to become one," explains the researcher. "This is the direction of our research - to try to locate and understand what is the particle size of the metals that causes catalytic activity and why."

In his new study, the team of scientists advanced a step in tuning catalysts that would have desired properties by demonstrating, for the first time ever, how the size of the catalytic metal nanoparticles anchored to the surface affects not only their level of catalytic activity, but also their electronic properties.

"Chemical catalysts are an important part of the world economy," notes the researcher. "Catalysts are used in virtually every industrial process, from the preparation of fuel and polymers to the reduction of impurities and rocket propulsion."

Catalysts are used in about ninety percent of the industrial chemical processes in the US and in the preparation of more than twenty percent of all industrial products, and these processes consume large amounts of energy.

In addition, industry as a whole produces about twenty-one percent of the greenhouse gas carbon dioxide emissions in the US alone, including three percent originating from the chemical industry. Thus, increasing the efficiency of the catalysts is "a key factor for both energy saving and reducing the emission of greenhouse gases," the researchers note. Catalysts are also used in the pharmaceutical industry, in the food industry, in the production of fuel cells and fertilizers, in the conversion of natural gas, coal or biomass into liquid fuels, and in chemical systems to reduce pollution and improve combustion in energy production.

The North American Catalyst Association says that catalysts contribute to thirty-five percent of global GDP (gross domestic product). "The bulk of this contribution comes from the production of high-energy fuels (gasoline, diesel, hydrogen), which are essentially based on the use of tiny amounts of catalysts in oil refineries," the researchers explain. "The development of cheaper and more efficient catalysts is essential for the storage, conversion and storage of energy," explains the head of the chemistry department at the University of Utah. "This research is essential to the nation's energy security."

Many important catalysts - such as those found in a catalytic converter that reduces the pollutant emissions of vehicles - are composed of metal particles that range in size from microns to nanometers. As the size of the metal particles of the catalyst gets smaller and smaller towards the nanometer range, their properties remain, initially, as they were originally. However, when the size decreases to about ten nanometers - containing about ten thousand atoms of the catalyst - the movement of the electrons in the metal is limited and restricted, so that their internal energy increases.

When there are less than a hundred atoms in the catalyst particles, the changes in size also cause instability in the electronic structure of the catalyst atoms. This instability significantly affects the ability of the particles to be used as catalysts, explains the researcher.

Previous experiments have shown that the chemical and electronic properties of the catalysts are affected by the size of the catalyst particles suspended in the gas. However, these isolated catalyst particles are quite different from the existing catalysts due to a metal oxide surface - the true form in which catalytic metals exist in industrial catalysts.

Previous experiments with surface-anchored catalysts often included a wide range of particle sizes. So these experiments failed to investigate the question of how the electronic properties and catalytic activity of the catalysts change depending on the size of the individual particles.

In the new study, the researchers used palladium particles of a defined size anchored by titanium oxide and used to convert carbon monoxide into carbon dioxide. The study showed not only how the catalytic activity changes depending on the size of the particles, "however we were also able to link this size dependence with electronic changes observed in the catalyst particles," explains the researcher. Other scientists did predict that this was the case, but no one before us was able to prove it." The lead researcher points out that this is the first ever demonstration of the strong dependence that exists between the size and activity of a catalyst on a metal surface and its electronic properties.

The chemists directed a laser beam to vaporize palladium to create electrically charged metal nanoparticles in the gas carried by the helium stream. Electromagnetic fields are used to capture the particles and direct them through a mass spectrometer, which selects only the palladium particles of the desired size. These particles are further anchored by a single crystal of titanium oxide as a thin layer. In the next step, the researchers used a variety of measurement methods to characterize the physical, chemical, electronic and catalytic properties of these catalysts.

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